The question can be changed into a new form:
Which element has the most negative electron affinity, or attraction for electrons? halogens have the highest electron affinities, and thus are more attracted to the electrons in the Hydrogen atom than any element in their respective periods.
In this case all the following choices are in the same period, thus Cl or Chlorine is the answer as it is a halogen.
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Explanation:
Since, it is shown that the reaction has been reversed. Therefore, value of 
will become 
.
Hence, new 
= 
= 20
Also, the number of moles of each reactant has been halved. So, 
for the reaction 
will also get halved.
Therefore, = 
= 
= 4.47
As the value of 
is given as +39.0 kJ. So, it means that the reaction is endothermic in nature. So, energy of reactants will be more than the products. Hence, according to Le Chatelier's principle reaction will move in the forward direction.
As a result, will also increase with increase in temperature.
